New Prosthetic Foot Delivers a Vastly More Natural Stride

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New Prosthetic Foot Delivers a Vastly More Natural Stride

Sarah Reinertsen didn’t start running until after she lost her left leg above the knee. She’s been at it for 23 years, which makes her an expert on prosthetic limbs. You name it and she’s probably run on it, in events like the Boston Marathon and the grueling Ironman Triathlon. The record-setting athlete knows what works and what doesn’t, which is why Össur sought her help refining a prosthetic foot made specifically for runners.

By surrounding Reinertsen with high-speed cameras and motion control devices, the company’s engineers reshaped the Flex-Run foot to provide a more natural stride. They extended the carbon fiber blade at the toe and tweaked the signature C-shape to improve forward energy return and smooth the roll-over on each stride. Most notably, they added a removable sole, created by Nike, that fits snugly around the base and secures with plastic tabs.

The new model was released last month and is sold worldwide. Reinertsen started testing the new model two years ago and noticed improvements immediately.

“The redesigned foot gave me a stronger push off on the toe,” she said. “I also ran with the new foot wearing heart-rate monitors and other measuring devices so we could empirically confirm that I was getting a better reaction from the foot while my heart rate was lower.”

In other words, Reinertsen could run more efficiently. That is to say, more naturally.

For all the advances of the Flex-Run, the addition of a replaceable sole may seem inconsequential. But it’s actually a big improvement, and something of a metaphor for the advancements we’ve seen in prosthetics for athletes. It wasn’t that long ago that Reinertsen was ripping apart running shoes and gluing the soles on her prosthetic foot. She saw competitors using things like bicycle tires, a laborious process that undermined training.

“When the sole wore down you would have to scrape off the old sole, rip apart another shoe and glue the new sole on,” she said. “I would often lose a couple days of training while scraping off and waiting for the new glue to set.”

Her story isn’t unusual among amputees trying to stay active. Reinertsen was born with proximal femoral focal deficiency. A bone-growth disorder led to her losing her leg above the knee at age 7. After meeting an amputee runner and being introduced to the Paralympic Games, Reinertsen threw herself into competition. She started setting national and world records in sprints – breaking the 100-meter world record for female above-the-knee amputees at age 13 – and ran on the U.S. Disabled Track Team for more than seven years. She’s completed seven marathons and in 2005 became the first female above-the-knee amputee to compete in the Ironman.

The prosthetic technology available in her early years was limited. Reinertsen began running on her everyday prosthesis, which featured an immovable rubber foot. A carbon fiber prosthetic made running easier, but it wasn’t built specifically for competitive use.

Three years later, in 1992, Reinertsen upgraded to a prosthetic made specifically for meets. It featured a J-shape blade that put the weight on the toe like a sprinter. It was an improvement, but it came with a drawback: Spikes. They were glued to the bottom, like cleats. They worked fine on a track, but Reinertsen wanted to compete in 5k and 10k road racing. She went back to the original model for road races until a foot designed to withstand the pounding of distance running came out in 1999.

Back then, even the best prosthetics fell short. Athletes were forever customizing and modifying them, without much support from the medical community. Hilmar Bragi Janusson, an executive vice president of R&D at Össur, chuckles when recalling the words some doctors used when they saw the rising number of amputees pushing the limits of their prostheses in the mid-1990s. These doctors complained that patients "abused their prosthesis."

"'Abused,'" Janusson said. "That was the word they used."

The development of prosthetics specifically for athletes is a relatively new field. The earliest prosthetics were made to resemble a human leg, not necessarily work like one. They lacked any type of energy to help the person move forward.

The big breakthrough came in the early 1980s with Van Phillips’ invention of C-shaped prosthetic foot. From the beginning, Phillips used carbon fiber, as its strength and efficiency as a spring made it ideal. But its shape – inspired by the flexing of the pole used in pole vaulting and the curve of his father’s Chinese sword – was truly innovative. The design stored and released energy as the person moved on it, allowing for a more natural gait. It also absorbed shocks vertically, which protected the rest of the body from excessive jarring. It was the first prosthetic that allowed lifelike movement. Those C-shaped blades have been refined through the years, but they’re still the most common for active use.

Phillips’ invention invigorated the field of sports biomechanics. Scientists learned more about how the human body moves and began prescribing more specialized training for athletes. By analyzing the techniques of individual athletes, specific deficiencies can be identified and training regimens tailored to address them. Janusson said amputees with active lives were among those who benefited most.

"The interest in understanding movement is very different when you see a function missing," Janusson said. "It attracts scientists to these individuals because it's very obvious what it missing and what can't be replaced and what can be used. From that point of view, this group is interesting from a biomechanical perspective and provides insight into how the whole body is coordinating in movement and sport."

This brings us to the new Flex-Run. While the new model was in development, Össur sent prototypes to Reinertsen to test. She’d run two-mile loops with the different blades as engineers measured and captured data. She also worked closely with noted Nike shoe designer Tobie Hatfield to test materials and tread patterns.

“The new improved design has been a joy to run on,” Reinertsen said. “I really plan to put it to the test this year as I have several big races on the schedule for 2012: the NYC triathlon, the Ironman NYC and the NYC marathon.”

Even with the advancements of the past 20 years or so, there’s still work to be done. Studies have shown that when an amputee runs with carbon-blade prostheses, they use about the same metabolic energy as a person running with biological limbs. But when the pace slows to walking speed, amputees use a higher metabolic energy than someone walking on two legs. Until a prosthesis comes along that can reduce that extra effort, amputees face physical problems from the stress of everyday movement.

“If a prosthesis can't emulate walking, it will be mean greater forces on the skeletal system, it will mean more back pain and joint pain later in life, it will mean increased socket discomfort. It will mean a lower overall activity level in the patient, which may lead to cardiovascular disease,” said Hugh Herr, an associate professor and director of the Biomechatronics group at the MIT Media Lab. “There’s great societal value to developing limbs that allow a person to walk only. Once we're there, we can they attempt to extend it to running and emulating what the body is doing in running.”

Herr, whose legs were amputated below the knee in 1982 after a climbing accident, believes bionics - prostheses that emulate or augment a biological function - are the gateway to the next stage of innovation. He singled out three interfaces: mechanical, as in how bionic limbs attach to the body; electrical, or how the human nervous system communicates with the artificial nervous system in the prosthesis and receives sensory feedback; and behavioral, as in developing controllers that made a bionic prosthesis move as if it’s made of flesh.

We’re still a long way from that point, as Michael Chorost notes in this month’s issue of Wired. Researchers have for the past decade been on the cusp of creating a truly bionic prosthetic. And such innovation is desperately needed, as there are some 185,000 limb amputations annually. Researchers at Sandia National Laboratories, the University of New Mexico and the MD Anderson Cancer Center, have brought us another step closer with the creation of an artificial structure that can support tissue growth — successfully merging severed nerves with robotic limbs. But their research, like so much in the emerging field of bionic prosthetics, remains beyond reach for now.

Still, the possibility intrigues Reinertsen, and how it might play into her future.

“As an athlete, I put my body through a lot of punishment - swimming, biking, and running Ironman triathlons takes a lot of work, and while I'm very fit, I think about the inevitable aging of the body,” she said. “When I'm 70 years old and still rocking through life on a prosthetic I don't want to be using a walker, I want to stand tall and be able to walk, climb stairs and I know I will be using bionics or the latest technology available so I can still live a life without limits.”